Project Summary Tumor cells exploit mechanisms of immune regulation to evade detection and eradication by host defenses. Foxp3+CD4+CD25+ regulatory T cell (Treg)-mediated immune suppression is crucial for immune evasion by tumor cells and an obstacle for successful tumor immunotherapy. Hence, the ability to disrupt Treg function is of major therapeutic significance. Recent work by us and others revealed that the key Treg transcription factor, Foxp3 is subject to polyubiquitination-dependent posttranslational regulation. Particularly, we found the E3 ubiquitin ligase Stub1, which is induced in response to a range of stress signals, facilitates the degradation of Foxp3 providing a potential target for dynamic modulation of Treg suppression. In the current proposal, we are seeking to: 1) Dissect molecular signaling pathways involved in Stub1 expression and its post-translational modification; 2) Understand the consequences of physiological Stub1 induction and genetic deletion for Treg cell homeostasis, differentiation and function; and 3) Test pharmacological activators of Foxp3 ubquitination as novel immunotherapic strategies to undermine immune suppression in the cancer setting. These studies will expand our understanding of the mechanisms behind posttranslational Foxp3 regulation. Specifically, we will further explore pathways determining Stub1 activity and expression, including the previously unappreciated phosphorylation of the ligase by the kinase GS3K?. To this end, we will utilize biochemical approaches and well-characterized models of in vitro and in vivo Treg function to establish the consequences of ablating these pathways. Furthermore, pharmacological modifiers of the Stub1/Ubiquitin-dependent pathway for Foxp3 degradation (identified in a drug screen and previous studies) will be tested for efficacy as breakers of immune suppression - a major obstacle for anti-cancer immunotherapy. This vetting will be carried out in an aggressive murine melanoma model (in vivo) as well as in ex vivo studies of human leukocytes obtained from healthy donors and advanced cancer patients. In so doing we will determine the potential therapeutic application of modulating Stub1 activity to boost anti-tumor immunity. Our experiments may reveal novel modes of regulating Stub1 activity and Foxp3 protein downregulation. Detailed assessment of physiological Stub1 induction and its impact on Foxp3 and Treg function is predicted to demonstrate a potent therapeutic application. Use of Stub1- activators in combination with proven checkpoint targeting agents may yield even better anti-tumor efficacy.